Stretchable Skin-Conformable Bioelectrodes Demonstrate Promising Capabilities for Healthcare Wearables

The adoption of wearable electronics that continuously monitor biosignals has revolutionized the healthcare and fitness sectors. These devices are being increasingly used and are expected to achieve a market value of around USD 572.06 billion by 2033. With their rapid proliferation, there is a growing need for high-quality bioelectrodes that can reliably record biosignals for long durations. Current materials used in bioelectrodes, such as metals, conductive polymers, and hydrogels, face challenges. They typically lack sufficient flexibility to stretch with the skin without breaking and are often not very permeable to humidity, which can lead to sweat accumulation and discomfort. A new type of bioelectrode material now promises to make wearable devices more comfortable and durable for both healthcare and fitness uses.

In a study published in the journal NPG Asia Materials on June 20, 2024, a research team from the Tokyo Institute of Technology (Tokyo, Japan) has introduced a bioelectrode material that is both stretchable and humidity-permeable, conforming closely to the skin. This advanced material features layers of conductive fibrous networks made of single-wall carbon nanotubes (SWCNTs) embedded in a stretchable poly(styrene-b-butadiene-b-styrene) (SBS) nanosheet. The nanosheet conforms tightly to the skin for accurate biosignal recording, while the carbon nanotube fibers enhance the material’s stretchability and humidity permeability.

The team applied SWCNTs as aqueous dispersions to coat SBS nanosheets, achieving a final thickness of only 431 nm after multiple layers were applied. Each additional layer of SWCNTs thickened and densified the fibers, altering the bioelectrode’s properties. Despite increasing stiffness with additional layers of SWCNTs—from an initial elastic modulus of 48.5 MPa to 60.8 MPa with one layer, and up to 104.2 MPa with five layers—the bioelectrode remained highly flexible. For comparison, pristine SBS nanosheets and those with one to three layers of SWCNTs (SWCNT 3rd-SBS) could stretch to 380% of their original length before undergoing permanent deformation, significantly outperforming metal electrodes like gold, which generally have Young's moduli in the several-hundred-GPa range and are capable of only stretching less than 30% of their original length before breaking

The durability of the bioelectrode material was also rigorously tested. The researchers immersed the bioelectrodes in artificial sweat and subjected them to repeated bending to measure changes in resistance. They observed only a minor increase in resistance, by about 1.1 times in sweat and 1.3 times over 300 bending cycles. Moreover, the SWCNT 3rd-SBS nanosheets exhibited minimal detachment after being rubbed ten times, affirming their suitability for long-term use. In practical applications, the researchers tested an SBS nanosheet with three layers of SWCNTs against conventional Ag/AgCl gel electrodes by attaching them to the forearm and measuring surface electromyography (sEMG) during gripping actions. The performance of the SWCNT-SBS nanosheet was on par with that of commercial electrodes, achieving similar signal-to-noise ratios of 24.6 dB and 33.3 dB, respectively.

“We obtained skin-conformable bioelectrodes with high water vapor permeabilities, which showed comparable performance in sEMG measurements to those of conventional electrodes,” said Associate Professor Toshinori Fujie from the Tokyo Institute of Technology, highlighting the material’s promising capabilities for healthcare wearables.

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Tokyo Institute of Technology